The present disclosure relates to methods for producing fluoride compounds and, particularly, methods for producing silicon tetrafluoride by acid digestion of fluoride salts of alkali metal or alkaline earth metal and aluminum.
Silane is a versatile compound that has many industrial uses. In the semiconductor industry, silane may be utilized for deposition of an epitaxial silicon layer on semiconductor wafers and for production of polycrystalline silicon. Polycrystalline silicon is a vital raw material used to produce many commercial products including, for example, integrated circuits and photovoltaic (i.e., solar) cells that may be produced by thermal decomposition of silane onto silicon particles in a fluidized bed reactor.
Silane may be produced by reacting silicon tetrafluoride with an alkali or alkaline earth metal aluminum hydride such as sodium aluminum tetrahydride as disclosed in U.S. Pat. No. 4,632,816 which is incorporated herein by reference for all relevant and consistent purposes. Production of silane may result in several by-products such as various fluoride salts of alkali metal or alkaline earth metal and aluminum (e.g., NaAlF4, Na5Al3F14 and Na3AlF6). Conventionally, these waste products are sold at low prices or are disposed of in a landfill.
Aluminum trifluoride is a versatile material that may be used as a component in an electrolyte melt for production of aluminum and may be used in various fluorination reactions. Aluminum trifluoride is conventionally produced by reacting hydrogen fluoride with relatively expensive alumina or alumina trihydrate. Silicon tetrafluoride is also a versatile material that may be used to produce silane or various halosilanes and can be used for ion implantation, plasma deposition of fluorinated silica, production of pure silica or of silicon nitride and may be used as a metal silicide etch.
A continuing need exists for methods to reuse the wastes produced during silane production to reduce the amount of material that must be landfilled or cheaply sold and to improve the economics of producing silane and resulting commercial products (e.g., photovoltaic cells). A need also exists for methods for producing valuable raw materials such as aluminum trifluoride and silicon tetrafluoride.